i think that the keyword of your question is specific heat capacity, when you apply heat to different materials you will get a different result, because the molecular structure of each one will allow the movement of the mols with a different freedom.

Substances with low specific heat such as metals require less input energy to increase their temperature. Substances with high specific heat such as water require much more energy to increase their temperature. The specific heat can also be interpreted as a measure of how well a substance preserves its temperature, i.e. "stores" heat, hence the term "heat capacity".

If you tell us what you are using this heat absorber for, in more detail, we can give you a complete answer.

More often than not, the quantity of importance is the thermal time constant which is the ratio of the heat capacity to the thermal conductivity...but without knowing the specifics of the design and the objectives, you really can not get even a half-good answer.

Well it's for use in a fuel cell. I want to trap the waste heat in a special chamber. Inside that chamber there would be a heat absorber.

There's a chart on a website that said that helium had the highest heat capacity, hydrogen excepted. But what you say is also true, if I could get something that took up a small amount of space but had high heat capacity, that would be perfect. How about a metal?